Scanning electron micrograph of stomatal rows from a modern limber pine needle.

One of the greater contributions of packrat midden research is the rich archive of plant and animal remains that can now be mined for various paleoclimate studies. The density of limber pine
needle stomata (pores that permit a plant to exchange gases with the atmosphere) from fossil packrat middens decreased 17% during the last deglaciation, coinciding with a 30% increase in atmospheric CO2.

Knowledge that atmospheric CO2 increased
30% during the last deglaciation and again since industrialization has stimulated research linking this greenhouse gas with past, present and future climate change. This knowledge also has inspired a host of field and laboratory experiments that
simulate plant responses at glacial (ca. 180-200 parts per million in volume, ppmv) to more than twice modern (>700 ppmv) CO2 levels. Beginning in the late 1980s, the scope was expanded to include measurement of stomatal density and carbon isotopes in
leaves from historical plant material, ranging from herbarium specimens collected during the past 200 years to leaf compressions in late Miocene and Pliocene clays of the lower Rhine Embayment in Europe.